This invention relates to the construction of an array of detectors suitable for imaging scenes emitting electromagnetic radiation and, more particularly, to the construction of a composite structure of a detector array and a semiconductor readout chip formed as a laminate with resilience to thermal expansion so as to permit thermal cycling for cryogenic operation without danger of inducing failures in metallic contacts between detector elements and the readout chip.
A detector array of particular interest is employed in the imaging of scenes emitting infrared radiation. Such detector arrays are operated at cryogenic temperatures, such as liquid nitrogen, during the detection of infrared radiation. Thus, there is always present a cycling of temperature between intervals of use and non-use of the infrared detector array. Such temperature cycling introduces expansion and contraction of components of the detector array, as well as in a semiconductor readout chip which is generally connected both physically and electrically to detectors of the array for extracting electrical signals from the detectors in response to the incident radiation.
One common form of construction of the infrared detector array provides for an electrically insulating substrate, such as a substrate of cadmium-zinc-telluride, upon which are grown epitaxially a P-type layer and an N-type layer of mercury-cadmium-telluride. The P-type and N-type layers of the mercury-cadmium-telluride provide a PN junction responsive to infrared radiation for introducing a current which varies in response to intensity of the radiation. The current is detected by circuitry of the readout chip. A composite construction of the laminate of the detector layers with the readout chip includes metallic contacts, typically of indium, which are located on both the detector array and the readout chip at the sites of terminals of the individual detector elements. As a practical matter in the construction of the indium contacts, the respective sets of contacts of the detector array and the readout chip are cold-welded together to form a permanent electrical and physical bond between the detector elements and the circuitry of the readout chip.
A problem arises in that the coefficients of thermal expansion of silicon, generally used in construction of the readout chip, the layers of the photodetector material and the substrate layer differ so as to introduce sufficient differential displacement between the indium contacts of the detector elements and the indium contacts of the readout chip to stress these contacts to the point of rupture. As a result, care must be employed in an environment of thermal cycling which may occur during use of the detector array so as to reduce a tendency to rupture. However, in spite of such care, contact rupture does occur with a resulting impairment of the utility of the detector array.